There have been "multiple fatalities" after a shallow 6.3 magnitude earthquake in Christchurch this afternoon caused buildings to collapse, police have confirmed.

Police said fatalities had been reported at several locations and that two buses had been crushed by falling buildings.

Christchurch resident Jane Smith, who works in the central city, told the Herald a work colleague had just returned from helping rescue efforts after a building facade had collapsed on a bus on Colombo St.

"There's people dead. He was pulling them out of a bus. Colombo St is completely munted."

TV3 reported that a person had died in the Christchurch suburb of Sumner.

Police said there were reports of fires in buildings in the central city and of people being trapped.

Police said all available staff were helping with the rescue operation and the Defence Force had been called in to assist.

Triage centres have been established for the injured at Latimer Square in the central city, Spotlight Mall in Sydenham and Sanitarium in Papanui.

A Herald reporter said that emergency services were struggling to enter the central city and were having to manoeuvre slowly around gridlocked traffic.

Shallow quake
GNS Science said today's quake was centred at Lyttelton at a depth of 5km at 12.51pm.

GNS said the earthquake would have caused more damage than the original 7.1 earthquake on September 4 because of its shallow depth.

Its data centre manager Kevin Fenaughty said residents said the quake's epicentre was located in the "worst possible location" for the city.

"It's a nightmare. A lot of people were just getting back on their feet after the original quake."

Another earthquake of 4.5 struck at 1.21pm, 10 km east of Diamond Harbour.

Streets flooded
Herald reporter Jarrod Booker said the shake lasted approximately one minute and was extremely violent - rocking buildings back and forth.

He said people had left buildings and were out on the streets where tarmac had cracked and water mains had burst, causing extensive flooding.

Tuam Street had become a river as water poured from ruptures in the road and was impassable in places.

The whole central city was in grid lock as people tried to evacuate central businesses to check their homes, Jarrod Booker said.

Most traffic lights were out and cars were also having to negotiate around hordes of people on foot.

Jarrod Booker said that he could hear sirens but that it would be difficult for emergency services to access the city because of the gridlock.

"Even sitting in a car you can feel continual shaking on a smaller scale than the original quake," he said.

Some pedestrians were standing on the footpaths and staring into space, apparently in shock.

'Great confusion'

Mayor Bob Parker said he was "thrown quite a distance" by the earthquake.

"That was, in the city central anyway, as violent as the one that happened on the 4th of September," he told Radio New Zealand.

Mr Parker said there were scenes of "great confusion" on the streets, also saying the roads were jammed as vehicles sought to get out of the central city.

"I know of injuries in my building and there are unconfirmed reports of serious injuries in the city."

Mr Parker did not know the extent of damage to the city's infrastructure, but advised people not to drink the water supply.

"We've been through this before this once, we now need to think we did at that time."

Buildings collapsed

Jarrod Booker said Christchurch's historic cathedral of the Blessed Sacrament on Barbadoes Street had half collapsed, with the remaining part of the building filled with cracks.

There was huge damage to other older buildings with large amounts of debris falling to the ground, he said.

He said the carpark at the Christchurch Star had turned into a river with huge cracks and that the roads had risen in areas.

People were comforting people outside amid a general state of shock as they tried to absorb what had happened, he said.

Radio New Zealand reported widespread damage to the city centre, with a church on Durham St collapsed and concrete lifted by up to a metre.

TV3 reported the Provincial Chambers Building had collapsed and it was believed people were trapped inside.

A listener told Newstalk ZB that the Piko Wholefoods building on Kilmore Street near the city centre, which was hit in the September 4 earthquake, was now "practically non-existent".

The spire on the Christchurch Cathedral had also collapsed.

A man said he and 19 other colleagues are trapped in Christchurch's Forsyth Barr building on Colombo Street.

Gary Moore told NZPA workers were stuck on the 12th floor as the stairwell had collapsed. He was not sure if people were trapped on other floors.

People were in a state of shock but were not injured and he urged NZPA to let somebody know.

He described the first quake and the aftershocks as catastrophic.

"We watched the cathedral collapse out our window while we were holding onto the walls."

"Every aftershock sends us rushing under the desks. It's very unnerving but we can clearly see there are other priorities out the window. There has been a lot of damage and I guess people are attending to that before they come and get us," he said.

A Newstalk ZB reporter in Christchurch said liquefaction was spewing out of the ground at St Albans High School.

School children had to be removed from the fields with liquefaction also spewing from the tennis courts.

Civil Defence response

Speaking to media at the Beehive's National Crisis Centre, Director of Civil Defence John Hamilton said a response plan was now being put together using all available national resources.

"That includes extra fire people, extra police personnel, assets from the Defence Forces. International offers of assistance are coming through from Australia in particular."

Mr Hamilton said the earthquake was a level three crisis - the highest for a localised event.

Phone lines are down and calls are not being connected to emergency services. Telecom said it is working to understand which services have been affected by the earthquake and get these restored as soon as possible.

Flights into Christchurch have been put on hold while Christchurch Airport checks the state of its runway. So far, four international flights have been diverted to Wellington Airport.

Today's quake was shallower and closer to Christchurch than the original Darfield quake, which took place 30km west of the city at a depth of 33kms.

Civil Defence advice

The Civil Defence has issued the following advisory:

Check yourself first for injuries and get first aid if necessary before helping injured or trapped persons.

Assess your home or workplace for damage. If the building appears unsafe get everyone out. Use the stairs, not an elevator and when outside, watch out for fallen power lines or broken gas lines. Stay out of damaged areas.

Look for and extinguish small fires if it is safe to do so. Fire is a significant hazard following earthquakes.

Listen to the radio for updated emergency information and instructions.

Do not overload phone lines with non-emergency calls.

Help people who require special assistance - infants, elderly people, those without transportation, families who may need additional help, people with disabilities, and the people who care for them.

Detailed safety advice will come from local authorities and emergency services in the area. People should act on it promptly. MCDEM, local civil defence authorities and scientific advisors are closely monitoring the situation.